1. Field of the Invention
The present invention relates to a battery pack having a battery protection monitoring circuit and to an electronic apparatus using such a battery pack.
2. Description of the Related Art
A conventional battery pack having a battery protection monitoring circuit, and an electronic apparatus using such a battery pack will be described by referring to FIG. 1. In this figure, a conventional battery pack 100 is detachably attached to a main unit 200, such as a portable information terminal, and supplies electric power to the main unit 200.
The conventional battery pack 100 has a chargeable and dischargeable battery 300, a protection circuit 400 for protecting the battery 300, and a management circuit portion 500 for posting a state of the battery 300 to a main unit 200 by using firmware.
The protection circuit portion 400 has an overcurrent detecting circuit 130 for detecting an overcurrent, an overcharge detecting circuit 150 for detecting overcharge of the battery 300, an overcurrent/overcharge protection circuit 140 for protecting the battery 300 from overcurrent and overcharge conditions, and a temperature detecting circuit 160 for detecting a temperature in the battery pack 100. The management circuit portion 500 has a microcontroller unit (MCU) circuit 180 for controlling each of the detecting circuit and the protection circuit and for posting the state of each of such circuits to the main unit 200.
A system power supply control portion 600 of the main unit 200 is operative to supply electric power POW fed from the battery pack 100 to the system of the main unit and to post the power state of the battery pack 100 to the system.
Protection devices of the conventional battery pack 100 illustrated in FIG. 1 are connected to the battery 300 and always continue to perform necessary minimum operations. Thus, the conventional battery pack 100 consumes electric power as part of self discharge thereof, though an amount of power consumption is low. Moreover, even when the battery pack 100 mounted in the main unit 200 is not used, the protection circuit portion 400 and the management circuit 500 of the battery pack 100 continue to perform so as to clearly indicate the presence of the battery pack 100 and the possibility of use thereof.
Therefore, when a battery pack is singly distributed, and displayed in a shopwindow, and is in stock or storage, the battery pack is gradually discharged and finally becomes dead. Further, even when a main switch of a main unit is turned off, a battery still may become exhausted. It is necessary in order to avoid the occurrence of such a situation to reduce the amount of the self-discharge of an unused battery as much as possible.
When the terminal voltage of a battery pack is almost 0, it cannot be expected that the protection circuit portion 400 and the management circuit portion 500 of the battery operate normally. Even in the case of such a battery pack, it is necessary that the battery pack can be safely charged by determining the state thereof. That is, it is necessary to easily discriminate battery packs, which cannot be used or are dangerous even when recharged, from normal battery packs, which can be used by being recharged, among battery packs that have lowered terminal voltages, and whose histories are unknown. Further, in the case that the terminal voltage of the battery 300 is almost 0 V and that the battery 300 is in an overdischarged state, it cannot be expected that the protection circuit portion 400 and the management circuit portion 500 operate normally. Thus, the battery 300 is thereafter charged under the control of the system power supply control portion 600 until the management circuit portion 500 starts to operate normally. That is, the battery 300 is charged without the protection function of the battery pack 100. Therefore, the system power supply control portion 600 needs to have the function of measuring a charging current and a pack voltage. However, the accuracy in measuring the charging current and the pack voltage in the main unit is poor owing to the fact that the ground floats relative to the battery voltage. Moreover, such a measuring function overlaps with the function of the circuit in the battery pack 100. This results in an increase in the cost of the battery pack.
There is further a danger of smoke emission and fire occurrence, resulting from a failure, owing to the fact that the battery 300 stores a large amount of energy. Thus, a safety mechanism is employed. For example, an internal-pressure-rise relief valve for relieving an internal pressure rise due to a circuit opening, which is caused by an overcurrent, and extraordinary temperature rise is provided in a battery cell itself. The battery pack 100 has an output short-circuit protection function and an overcharge protection function. The system power supply control portion 600 has an abnormal charging control protection function and a temperature anomaly protection function.
Various detecting circuits, such as the overcurrent detecting circuit 130 and the overcharge detecting circuit 150, are provided in the protection circuit portion 400. These circuits perform detection operations by using comparators. The overcharge detecting circuit 150 of the conventional protection circuit portion 400 has a single threshold value for detecting an overcharge. Moreover, the overcurrent detecting circuit 130 thereof employs a single threshold value for detecting an overcurrent.
In the case that the threshold for detecting an overcharge has a single value, even when an overcharge is normally detected, the charging of the battery is continued if no protection (or control) means operate normally according to a result of the detection of an overcharge. Thus, the charging of the battery proceeds, so that in a worst case, there is a danger of emission and fire. Mechanical measures, such as an internal pressure relief valve, may be taken in each of the battery cells. It is, however, important for protecting a battery, which is adapted to store large energy, to improve the reliability thereof by doubling the protection means. In this case, the dual protection realized by employing different protection means is more effective in improving the safety of the system than that realized by employing the same protection circuits. In the case that the conventional battery pack is adapted to have two systems having protection functions, it is necessary to prevent both systems from mutually interfering with each other to thereby impede normal operation of the battery pack.
Further, in the case that the threshold for detecting an overdischarge has a single value, even when an overdischarge is normally detected, the discharging of the battery is continued if no protection (or control) means operate normally according to a result of the detection of an overcharge. Thus, the discharging of the battery proceeds, so that in a worst case, an irreversible reaction may proceed in the battery so that the battery cannot be recharged. Further, when the battery pack enters a discharge region, a drop in the terminal voltage increases with the use of energy. This may suddenly impede an operation of the system. It is, thus, important for ensuring a stable operation of the system to improve the reliability thereof by doubling the protection means. In the case of protecting the battery from an overdischarge, similarly to the case of protecting the battery from a overcharge, the dual protection realized by employing different protection means is more effective in improving the safety of the system than that realized by employing the same protection circuits. In the case that the conventional battery pack is adapted to have two systems having the protection functions, it is necessary to prevent both systems from mutually interfering with each other to thereby impede normal operation of the battery pack.
In the conventional battery pack 100 illustrated in FIG. 1, the protection circuit portion 400 and the management circuit portion 500 can be constituted by one or more LSIs. Further, the protection circuit portion 400 and the management circuit portion 500 may be constituted by different LSIs, respectively. In this case, monitor data represented by an analog signal sent from each of the detecting circuits is received at an analog-to-digital conversion (ADC) input portion having an A/D conversion function in the MCU circuit 180. Status information represented by a digital signal is received at an input/output (I/O) port thereof. Thus, various kinds of monitor data and status information are outputted from the protection circuit portion 400 to the management circuit portion 500. This is a primary factor of an increase in the number of input/output pins of each of the LSIs.
An erroneous detection may occur in each of the various kinds of detecting circuits of the protection circuit portion 400 owing to an abrupt change in load, a transition current at the time of attachment of each of the detecting circuits to the main unit 200 or detachment of each of the detecting circuits therefrom, and impulse noises thereto. Even when a method of limiting a band width of detected signals by connecting a resistor R and a capacitor C thereto to thereby constitute a low-pass filter (LPF) is performed as a countermeasure to an error detected in this way, this method has drawbacks in that the value of CR increases because the frequencies of a necessary region are too low, that, the detecting circuits thus cannot be incorporated into an LSI, and that, when the capacitor is externally installed, the number of pins of the LSI increases. Further, although there is need for sending status and monitor data detected by the protection circuit portion 400 to the MCU circuit of the management circuit portion 500, it is desired to enable the sending of multiple status and monitor data to the MCU circuit without increasing the number of pins of the LSI and the size of the control circuit.
A method of detecting a rise in the temperature of the battery 300 due to an overcharge and of stopping the charging thereof is used so as to protect the battery from an overcharge. This method, however, has drawbacks in that the accuracy thereof is poor and that the response time is long. Moreover, a method of detecting a change in a battery voltage, which is caused by an overcharge, a rate of such a change, a variation in electric current, and a change in the internal impedance, is also used. This method, however, has drawbacks in the poor accuracy thereof and in the complex constitution of the circuit.
A method of detecting a decrease in the voltage of each of the cells of the battery 300, which is caused by an overdischarge, and of stopping the discharging thereof has been used so as to protect the battery from overdischarge. This method is adapted to detect that among a plurality of cells, the cell voltage of one of the cells is lower than a predetermined threshold value. Thus, even if the battery pack 100 still has available electric power, when the discharge occurred in a part of the cells is detected, the entire battery pack 100 is regarded as being overdischarged. Therefore, this method has a drawback in that the available electric power of the battery pack 100 cannot be fully used. In contrast, generally, a method of monitoring the overdischarge voltage of the entire battery 300 instead of monitoring the overdischarge voltage of each of the cells uses the monitoring means in common at the time of charging the battery 100 and at the time of discharging thereof. Consequently, it is inevitable for avoiding the danger at the time of overcharging a lithium battery to monitor each of the cells. Thus, this method cannot be employed.
Even if the battery pack 100 is constituted by selecting battery cells so that the terminal voltages of the battery cells, all of which are new articles, are equal to one another, a difference is caused in terminal voltage among the cells owing to a large difference in charging/discharging deterioration characteristics among the individual cells when the use of the battery 100 is continued. This results in occurrence of a drawback in that the available power of the battery pack 100 cannot be fully used.
Usually, the threshold for detecting an overdischarge is set at a rather high value so as to ensure a stable operation of the system during the discharging of the battery. In this case, some battery packs 100 may be detected as being overdischarged, even when such battery packs still have sufficient available capacity.
Further, there has been developed a method of monitoring the voltage and current of the battery 300 and managing the remaining capacity so as to ensure a stable operation of the system when overdischarged. As shown in FIG. 1, this method uses the MCU circuit 180. Thus, there is a possibility that bugs may occur in the firmware (or software) and that a runaway of the MCU 180 may occur. Consequently, this method has a drawback in that it is difficult to ensure reliable protection.
In the case that an overcurrent, whose magnitude is outside a normal operation range, flows in the battery pack 100, there is a necessity for surely protecting the battery pack 100 without damaging the circuit and parts thereof. A method of detecting a current flowing through the battery 300 and turning off a switch, which is usually constituted by a field effect transistor (FET) and provided on a charging/discharging path, in the case in which the detected value of the current exceeds a predetermined value, is employed so as to protect the battery pack 100 against an overcurrent. There is another method of protecting the battery pack 100 by causing a fuse to be blown by an overcurrent. However, after such an operation, the blown fuse must be replaced with new one. Thus, it is desired that the battery pack 100 have the function of protecting the battery 300 before the fuse blows.
Moreover, there is the need for allowing the protection function to be performed without being affected even at an occurrence of a malfunction of the managing/monitoring function of the management circuit portion 5000 of the battery pack 100 to thereby ensure the safety thereof. For example, when the value of a comparison voltage set in the overcurrent detecting circuit 130 or the overcharge detecting circuit 150 becomes abnormal owing to bugs occurring in the firmware of the MCU circuit 180, the detection cannot be correctly achieved. An occurrence of such a situation should be prevented. Furthermore, there is the need for preventing an occurrence of a situation, in which the charging or discharging of the battery cannot be achieved due to bugs occurring in the firmware of the MCU circuit 180 even if the protection circuit portion 400 operates normally.
Additionally, it is necessary to accurately detect which of a charging direction and a discharging direction the direction of a flow of a pack current is. Especially, in the case of a low current, for instance, in the case that the main unit 200 is in a standby state or a sleep state, for the purpose of precisely discriminating between a charging state and a discharging state, it is necessary to accurately detect which of the charging direction and the discharging direction the direction of a flow of a pack current is.